The present invention relates to electrophotographic development generally and, more particularly, to a process and device for sensing the amount of residual toner in a developer forming electrostatic latent images during an electrophotographic developing process.
In general, a developing apparatus using an electrophotographic developing process such as a laser beam printer, copying machine, plain paper facsimile or the like, serves to form an electrostatic latent image with toner on an exterior circumferential surface of a photosensitive drum, thus playing an important role in determining the quality of the electrostatic latent image quality. If the amount of toner stored in the developing apparatus falls below a predetermined amount as a result of its consumption, a toner-empty state is indicated by a sensor. Devices for sensing the amount of residual toner of in conventional designs of electrophotographic developing apparatus are usually provided with a toner container on the top portion of a housing in the developing apparatus, and with an agitator for preventing toner cohesion in the bottom portion of the housing. The agitator has arms for conveying toner to a developing roller. Typically, a piezoelectric sensor is positioned at the bottom inside portion of the housing to sense the amount of residual toner beneath the agitator. A blade is installed at the top portion of the developing roller to control the height, and thus the quantity of the toner deposited onto the developing roller. A photosensitive drum is located adjacent to the developing roller.
While the toner is supplied from a toner container to the interior of the housing, the rotation of the agitator prevents; the toner from becoming cohesive and conveys the toner from the interior of the housing to the developing roller. The toner is attracted to the developing roller by a magnetic force. The toner is developed on the photosensitive drum by uniformly adjusting the height of the toner with the blade, which controls the height of the toner. During this process, the supply of toner in the housing where the agitator is installed is gradually depleted.
A method for sensing the amount of toner as stated above is disclosed in Takeda et al., U.S. Pat. No. 4,647,185 entitled Developer Detecting Device.
Designs incorporating these features are well known in the art. In order to sense when the supply of toner inside the housing is reduced to a quantity below a reference level, some currently available designs rely upon a piezoelectric sensor changing its output sensing signal level according to the weight of the toner stored in the upper portion. A central processing unit generates a display control signal for displaying a visual indication of a toner-empty state on a display unit in response to the sensing signal produced by the piezoelectric sensor. Other devices for sensing the amount of residual toner in a developing apparatus utilize a reed switch activated by a magnetic field.
An early design, represented by Nawata, U.S. Pat. No. 4,951,091, Image Forming Apparatus Having Toner Quantity Detection Means, describes a developing apparatus incorporating a reed switch and a magnet to sense when the supply of toner is low. The magnet is attached to a plate, which is normally surrounded by toner when the housing is full. A lever, also surrounded by the toner, influences the plate as toner is depleted from the developing apparatus. When a predetermined amount of toner remains, the magnet is no longer surrounded by toner and the lever forces the plate towards the reed switch. The reed switch is activated by the magnetic field produced by the magnet and an indication that the toner is low is conveyed to the user.
A more recent design such as that shown in Nishimura et al., U.S. Pat. No. 5,257,076, entitled Toner Feeding Device Capable Of Signalling Need To Replenish Toner, has a multichambered toner housing incorporating reed switch responsive to a magnetic field. To determine when toner needs to be replenished, a hinged plate with a magnet fixed to the plate rests on top of toner contained within the housing of the developing apparatus. As the toner in the housing is depleted, gravity forces the plate to fall with the diminishing supply of toner. After the plate falls by a predetermined distance, the magnet trips a reed switch positioned in the housing, to indicate that the toner needs to be replenished.
An improved method is disclosed in U.S. application Ser. No. 07/989,828, which is entitled as Device For Detecting Toner Using An Electrophotographic Machine, assigned to SamSung Electronics Co., Ltd. This improved method utilizes a metal plate which rides on the toner contained in a hopper. As the toner is depleted, the metal plate moves with the surface of the toner. Finally, as the toner is completely depleted the metal plate rests upon the bottom portion of the hopper and attracts a magnet attached to one end of a bell crank, situated below the hopper. As the magnet is pulled toward the hopper and metal plate, the other end of the bell crank covers a photosensor to detect an empty state of toner in an electrophotographic machine.
I have observed various deficiencies in conventional developing devices such as those described above. The piezoelectric sensor used to sense the amount of residual toner is generally high in cost, a factor that contributes to unwarranted restriction upon a manufacturer's market share due to uncompetitively high prices of the finished product; moreover, such sensors tend to be sensitive to environmental conditions. Additionally, due to its sensitivity, the piezoelectric sensor must be carefully placed and left undisturbed where the toner is sensed; this is an unweildly restriction upon a manufacturer's freedom of design that can result in a less than optimal configuration of the toner hopper. As a result of its location inside the hopper, the sensor creates a protuberance and inhibits toner from smoothly circulating through the hopper. In addition to these problems attributable to conventional designs, the piezoelectric sensor generates undesirable vibration and noise due to its own characteristics.
Moreover, the developing devices which use a reed switch or a photosensor utilize a separate plate to move with the toner and trip the reed switch or photosensor to indicate when toner is low. The separate plate may become jammed and uses space in the housing which contains the toner. Additionally, these conventional devices must utilize a separate means to agitate the toner in the housing.